This invention relates to an image processing apparatus, and in particular it relates to apparatus of the type which includes a number of channels of detectors and scans an image in swathes, each swathe comprising a simultaneous scan of all of the channels. Such systems are commonly used in infra-red and other radiation imaging systems.
However, since there are inevitably variations in detector element responsivities and offsets, a degree of signal processing is generally required to match the detector signals. Such a signal processing technique usually makes one or more assumptions, the most common of which is that the portions of the image viewed by each detector element during one entire field or frame scan are essentially the same, i.e., the average image intensity viewed by the first detector element over the entire frame or field is equal to the average intensity viewed by the n'th detector. For the majority of images this assumption is reasonable and good matching between channels is achieved.
However, for certain images these assumptions are not valid, particularly where there is a temperature gradient across a scene such as a transition between sky and ground. This results in a swathe related banding structure becoming evident in the displayed image in which bands are seen on the image of width equal to the swathe width and the swathe boundaries are highly noticeable. The degree of degradation of the image will of course depend upon the actual scene being imaged.
It is convenient to think of these degraded images the sum of the ideal image and a so called "artifact" image which varies in the vertical image direction but substantially constant in the horizontal direction, the artifact image being that portion of the image which differs from the idealised image. Although the artifact image may show substantial differences from line to line within a swathe, due to differences in the gains and offsets of individual detectors, the artifact image of an individual channel often only varies slowly in the vertical image direction This results in the artifact portion of the image comprising a slowly varying structure that is repeated for each swathe in the image. Thus a banded structure is seen.
The present invention arose from a need to provide a method for removing the artifact and producing a clearer and better quality final image.